1. Field of the Invention
This invention relates to low temperature coefficient voltage output circuits, and more particularly to providing temperature compensation to a base circuit which produces a preliminary output voltage having a temperature coefficient of known polarity.
2. Description of the Related Art
Voltage reference circuits provide a fixed voltage for purposes such as regulated power supplies and comparison with other voltages. It is important that the output voltage of the reference circuit be made temperature invariant, to the extent practical. This is typically done by measuring the reference""s output voltage at high and low temperatures, and then trimming the circuit to compensate for changes in the output voltage between the two measurement temperatures.
In practice, multiple voltage references in integrated circuit (IC) form are typically provided on a single wafer, the voltage measurements at the high and low temperatures are taken at the wafer level, and the individual reference circuits are then trimmed to reduce their temperature dependency, also at the wafer level. Other circuits also require some form of temperature compensation to maintain a substantially temperature invariant voltage level. One example is the offset voltage for an operational amplifier.
There are several problems with this approach. First, the wafers are typically held on a chuck, and the chuck temperature is commonly directly sensed to obtain an indication of the wafer temperature. This requires that the chuck temperature at which the voltage is read be accurately known, which is difficult to do in a production environment. Furthermore, although the chuck temperature is generally close to that of the wafer, the two temperatures are usually not exactly equal, which leads to further inaccuracies. While the wafer temperature can be read directly, this is expensive to do. Also, it is necessary to keep track of which voltage and temperature measurements go with which die on a given wafer, and among the dies of different wafers. Again, this is difficult to do in a production environment.
The present invention seeks to provide a voltage output circuit with a low temperature coefficient (TC), and an associated temperature compensation method which eliminates the need for both accurate temperature measurement, and for keeping track of the different dies and wafers and their associate voltage and temperature measurements.
These goals are achieved with a voltage output circuit that includes a base circuit which produces a preliminary output voltage having a TC of one polarity, and a compensation circuit having a pair of current sources with different TCs that are connected to compensate for the TC of the base circuit""s output voltage (Vo) so as to yield a compensated Vo with a lower TC than the preliminary Vo TC. The differential between the currents of the two current sources is transformed to a voltage drop which has a TC of opposite polarity to the TC for the base circuit""s Vo. The voltage drop is combined with the base circuit Vo to yield the compensated Vo.
In a preferred embodiment, the voltage drop circuit is implemented with a resistor connected to receive a current that varies with the differential between the two current sources. The resistor produces a compensation voltage with a TC that approximates, but is of opposite polarity to, the preliminary Vo TC.
To provide a compensated Vo over a predetermined temperature range, the current sources are first trimmed at one temperature level within the range, preferably at one end of the range, so that they are substantially equal in absolute value and thereby cancel, producing zero compensation voltage. Then, at another temperature within the range, preferably at the opposite end of the range, the voltage drop resistor is trimmed to set the compensated Vo at the desired value.
One of the current sources can have a positive TC and the other a negative TC, or the current sources can both have positive or negative TCs, so long as the TC of one source differs from that of the other in a direction and by an amount that produces a compensation voltage of the desired polarity and magnitude. In the case of a voltage reference circuit, the compensated Vo is amplified by an output buffer amplifier. The amplifier is trimmed to produce a desired reference voltage at the first temperature setting, at which the current sources are set equal, and continues to provide the same or a close reference voltage as the temperature varies.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.